CHARLESTON, S.C. — Along much of the East Coast, hotel owners, tourism officials and would-be vacationers kept a watchful eye on forecasts Wednesday as Tropical Storm Arthur churned off Florida, threatening Fourth of July plans for thousands of people.

Arthur, the first named storm of the Atlantic season, was expected to strengthen to a hurricane by Thursday. Though early maps showed it was unlikely to make landfall in the U.S., it was forecast to skim the Outer Banks of North Carolina — a string of narrow barrier islands prone to flooding but popular for beachgoers — as a Category 1 hurricane Friday.

A swath of nearly 200 miles of North Carolina coast was under a hurricane watch Wednesday. A tropical storm watch was in effect for parts of Florida the Carolinas. There and elsewhere up the East Coast, forecasters predicted rain, winds, potential flooding, high surf and rip currents.

The worst of the storm should occur at Cape Hatteras, North Carolina, about dawn Friday, with 3 to 5 inches of rain and sustained winds up to 85 mph, said Tony Saavedra, a meteorologist at the National Weather Service. The storm should move through quickly and be off the coast of New England later in the day, perhaps making landfall in Canada’s maritime provinces as a tropical storm, he said.